A number of different measuring principles exist for fill level measurement. Vibronic limit level switches use a mechanically oscillatable unit, for example, in the form of a rod or a fork, which is placed at a fill level height to be monitored in a container and excited to execute resonant, mechanical oscillations. The frequency of the oscillations depends on whether the oscillatable unit is oscillating in air or covered by a liquid medium. A frequency change indicates, consequently, the reaching of the limit level. For detection of bulk goods, as a rule, a monitoring of the oscillation amplitude is used.
Another type of fill-level measuring device is based on the capacitive principle. In such case, a probe electrode is supplied with an alternating voltage signal and the capacitance between the probe electrode and the container wall or a second electrode is determined. For limit level measurement, also a coaxial arrangement of probe- and ground electrode is known for introduction through a wall of the container.
Especially for continuous fill level measurement in containers, further known are so called TDR probes. Such sensors operate according to a principle involving measuring the travel time of electromagnetic signals, which propagate along a waveguide protruding into the medium and are reflected at the interface of the medium. Such sensors are strongly susceptible to high-frequency disturbance signals. Known from Offenlegungsschrift DE 20016962 U1 is a limit level switch, which utilizes the TDR principle.